1. Field of the Invention
The present invention relates to a novel nonlinear optical material having an increased second harmonic generating ability and usable for an optical switch, an optical memory in an optical data/information processing or optical communication system, or an optical bistable element to be used in optical signal operation. Further, it pertains to a novel nonlinear optical crystalline material containing a salt obtained by the reaction of a conjugated aromatic carboxylic acid compound with an optically active amine.
2. Description of the Related Art
The nonlinear optical effect refers to, for example, a secondary or higher effect of the magnitude of an applied electric field exhibited by the relationship of an electric polarizing response of a substance, which is only primarily proportional to the magnitude of the applied electric field, when, for example, a strong photoelectric field such as a laser beam is applied to the substance.
A secondary nonlinear optical effect may include a second harmonic generation of converting the wavelength of an incident light to a 1/2 wavelength, a parametric oscillation which converts a light with one kind of wavelength into a light with two kinds of wavelengths, and a secondary light mixing which on the contrary generates a light with one kind of wavelength from a light with two kinds of wavelengths. Due to these various characteristics, materials having a nonlinear optical effect will be for use as an optical switch, optical memory in optical data/information processing, or such elements as optical bistable element, optical switch, etc. to be used in optical signal operations.
Generally speaking, in this field of the art, inorganic materials, primarily LiNbO.sub.3, have been studied and investigated, but inorganic materials had a drawback such that great difficulty is encountered in forming a desired optical element, because of such shortcomings as their performance indices which are not so great, small response speed, no good form workability, great hygroscopicity, low stability, etc.
In recent years, in contrast to these inorganic materials, researchers are becoming more interested in application of organic materials. This is because the response of organic materials is based primarily on the electron polarization, whereby the nonlinear effect is great, and also the response speed is great, as confirmed and reported in the art. For example, a large number of studies are reported in the ACS Symposium Series Vol. 233, 1983). The secondary nonlinear optical characteristics to be dealt with in the present invention, which is a third rank tensor, cannot be evoked if a symmetric center exists in the molecule or the crystal. For this reason, in the case of organic materials, even when they may have a structure exhibiting an excellent nonlinear effect at molecular level, they must be formed into crystals or solid state for using the second harmonic generation as the practical mode. However, at such stage of solidification, an inversion symmetrical structure will be frequently formed preferentially, whereby there has been involved the problem that a nonlinear optical effect as an optical element can not be exhibited.